This invention relates to fittings for conduits and especially for conduits used in ultra clean rooms. The advent of new technologies such as those associated with the semi-conductor industry imposed new demands on the systems which deliver or transfer materials employed in the manufacturing processes. For example, gasses and liquids used in the semi-conductor manufacturing industry must be substantially free from all particulate matter. It has been determined that one source of undesirable particulate matter is from fittings between conduits which transfer these gases and liquids. Past fitting designs were characterized by crevices which tended to retain particulate matter and later abruptly release particulate matter at unsatisfactory levels. For applications which subject fittings to vibrations, temperature fluctuations or other physical stresses, commercially available fittings often fail and their air tight seals are lost. It also has been difficult to determine when a proper seal is achieved by the components of these fittings, and testing for seal integrity has been done by trial and error. If severe pressure or unnecessary torque is imposed on the components of these fittings, the seal components become damaged and particulate matter can be introduced into the system through them. Accordingly, it is desirable to design a reliable fitting that is easy to employ which does not significantly contribute to the introduction of particulate matter into ultraclean.
A number of specialized fittings have been developed for use in conduit systems for ultraclean environments. One successful design involves the incorporation of an integral annular sealing bead on the radial end wall of a stainless steel tubular conduit member which engages a metal gasket. The metal gasket has an annular recess designed to receive the sealing bead. This initial design has been the subject of a number of improvements such as those disclosed in U.S. Pat. No. 4,854,597 to Leigh, and U.S. patent applications Ser. No. 07/652,225 of Raymond McGarvey for "Zero Dead Volume Fitting" and Ser. No. 07/694,066 of Raymond McGarvey for "Over-tightening Prevention Gasket". Improvements have included changes to the profile of the annular sealing bead, changes to faces of the gasket which receive the bead, and devices for over-tightening prevention and torque reduction.
A second successful fitting design, known as a knife-edge seal, is often used in special applications such as when forming vacuum joints. These fittings form a seal by the penetration of a sharp edge or corner which axially projects from the conduit member into a soft metal gasket. As the knife edge penetrates the gasket, the gasket deforms and a seal is formed. These seals have been successful in high temperature environments and, in fact, in many instances the seal is baked. These high temperature environments rule out the use of non-metallic sealing elements which, when subjected to high temperatures, can fail and release materials in the form of gasses that can contaminate an ultraclean system.
Although seals formed with annular sealing beads and seals having knife-edge elements have been used, both are subject to various limitations. Because the knife-edge permanently deforms the gasket, knife edge seals are not favored for applictions which require repeated sealing engagements. Therefore, seals usually have had annular sealing beads for these applications. However, seals formed with annular sealing beads are prone to failure, especially after repeated seals or when subjected to extreme conditions. Furthermore, leaks sometimes occur after installation because of wear, mishandling, and contact with electropolishing solution. Most of these fittings are welded into systems or integrally machined on the components and, thus, are difficult and expensive to replace.
Accordingly, any improvement in seal design which maintains the integrity of the seal and minimizes the incidence of particulate matter is a welcome advancement in the industry.